In one of the greatest moments in modern medical science, American microbiologist Jonas Salk on 12 April 1955 pronounced his newly invented polio vaccine safe and effective in almost 90% of cases. Salk had discovered that injecting a small amount of killed virus prompts the body to produce protective antibodies without triggering polio, a disease that has caused permanent paralysis, particularly in children, since the days of ancient Egypt.

Salk identified the three main varieties of poliovirus, developed a killed-virus vaccine, tested it in monkeys, and in May 1952 proceeded with a massive human field trial on more than 400,000 children--the largest medical experiment ever conducted in the United States. After success was proclaimed in 1955, Salk's polio vaccine was administered to more than 200 million people in the United States, and the incidence of polio dropped 96% by 1961. Salk's vaccine eventually was replaced by a more effective live oral vaccine developed by competitor Albert Sabin and distributed worldwide. Some 2000 cases of polio were reported last year, most of which were in the Indian subcontinent and the former Soviet Union.